ABSTRACT
Membrane proteins include mainly receptors, transporters and ion channels. All live cells rely on them to establish a resting transmembrane electrostatic potential and ion gradients across their membranes. Ion channels allow the movement of ions from one side of the membrane to the other along the electrochemical gradients. They may contain a membrane-traversing ion-conducting pore domain and regulatory domains within and/or outside of the bilayer. The pore domain becomes accessible to ions after allosteric changes in the regulatory domain(s), e.g., voltage sensing domains in voltage-gated ion channels, open it. Studies of membrane proteins represent a major challenge in membrane protein biochemistry, especially when quantitative analysis of protein-membrane interactions is desired. Despite past progresses in understanding membrane proteins at the protein level, dynamic effects of various native lipids on membrane proteins remain poorly defined at the molecular level. Technical limitations are the main roadblock. With available technologies, precise control of lipid composition and phase behavior is only achievable in reconstituted membranes in order to reach accurate, quantitative understandings of lipid effects on membrane proteins. This chapter will introduce multiple latest techniques for reconstituting membrane proteins into well-controlled lipid environments, using ion channels as examples. Because quantitative measurements of ion channel activity are important to investigating their roles in different human diseases, we will compare various methods used for measuring channel activities and will discuss possible solutions to overcome current technical limitations.
